High nitrogen steel (HNS) has an excellent tensile strength and impact property due to the solid solution strengthening of nitrogen, which provides a good application prospect in many fields. Fusion welding is one of the main processing methods of HNS, but the process is prone to spatters, serious nitrogen losses, N₂ porosities, and poor performances of joints, resulting in the failure of large-scale engineering applications of HNS. In this work, the development of HNS welding is reviewed, and the problems including the droplet transfer instability, N₂ porosities, nitrogen losses and poor mechanical properties due to high nitrogen content are discussed. According to previous welding experiences, the adoption of a welding method with a low heat input is proposed, which utilizes powders instead of wires, optimizes compositions in the shielding gas and feeding materials in order to solve the above problems, and improves the mechanical properties of the weld.

The titanium alloy components utilized in the aviation field are typically large in size and possess complex structures. By utilizing multiple additive manufacturing processes, the precision and efficiency requirements of production can be met. We investigated the hybrid additive manufacturing of Ti-6Al-4V using a combination of cold metal transfer additive manufacturing (CMTAM) and laser metal deposition (LMD), as well as the feasibility of using the CMT-LMD hybrid additive manufacturing process for fabricating Ti-6Al-4V components. Microstructural examinations, tensile testing coupled with digital image correlation and dynamic compressive experiments (by the split Hopkinson pressure bar (SHPB) system) were employed to assess the parts. The results indicate that the interface of the LMD and CMTAM zone formed a compact metallurgical bonding. In the CMTAM and LMD zone, the prior-β grains exhibit epitaxial growth, forming columnar prior-β grains. Due to laser remelting, the CMT-LMD hybrid additive zone experiences grain refinement, resulting in equiaxed prior-β grains at the interface with an average grain size smaller than that of the CMTAM and LMD regions. The microstructures reveal significant differences in grain orientation and morphology among the zones, with distinct textures forming in each zone. In the CMT-LMD hybrid zone, due to interfacial strengthening, strain concentration occurs in the arc additive zone during tensile testing, leading to fracture on the CMTAM zone. Under high-strain-rate dynamic impact conditions, the LMD region exhibits ductile fracture, while the CMTAM zone demonstrates brittle fracture. The hybrid zone combines ductile and brittle fracture modes, and the CMT-LMD hybrid material exhibits superior dynamic impact performance compared to the single deposition zone.